DownloadRangeToStreamAsync and File Save using ASP.NET and MVC5

Question

I am attempting to download a file from Azure Storage in the form of an CloudBlockBlob. I want to allow the user to select where to put the downloaded file, so I have written the following code to do this

[AllowAnonymous]
public async Task<ActionResult> DownloadFile(string displayName)
{
    ApplicationUser user = null;
    if (ModelState.IsValid)
    {
        user = await UserManager.FindByIdAsync(User.Identity.GetUserId());

        // Retrieve storage account and blob client.
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(
            ConfigurationManager.AppSettings["StorageConnectionString"]);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference(
            VisasysNET.Utilities.Constants.ContainerName);

        // If the container does not exist, return error.
        if (container.Exists())
        {
            foreach (IListBlobItem item in container.ListBlobs(null, false))
            {
                if (item.GetType() == typeof(CloudBlockBlob))
                {
                    CloudBlockBlob blob = (CloudBlockBlob)item;
                    if (blob.Name.CompareNoCase(displayName))
                    {
                        using (Stream stream = new MemoryStream())
                        {
                            await blob.DownloadRangeToStreamAsync(stream, null, null);
                            return File(stream, displayName);
                        }
                    }
                }
            }
            return RedirectToAction("Index", "Tools");
        }
    }
    return new HttpStatusCodeResult(HttpStatusCode.ServiceUnavailable);
}

This idea was to return the FileStreamResult and allow the browser to provide the "Save Dialog". However, when the controller posts to the view, I get an

Cannot access a closed Stream. Exception Details: System.ObjectDisposedException: Cannot access a closed Stream.

Now the problem is clear, however, how to circumvent it is not. I want to ensure the MemoryStream is released and disposed as it is likely to hold data up to ~10M. How can I dispose the MemoryStream but allow the FileStreamResult to be passed back and used by the browser?

Thanks for your time.

score 1 · Accepted Answer · edited May 23 '17 at 12:12

Simply don't dispose it. Have look at FileStreamResult implementation:

 protected override void WriteFile(HttpResponseBase response) {
            // grab chunks of data and write to the output stream
            Stream outputStream = response.OutputStream;
            using (FileStream) {
                byte[] buffer = new byte[_bufferSize];

                while (true) {
                    int bytesRead = FileStream.Read(buffer, 0, _bufferSize);
                    if (bytesRead == 0) {
                        // no more data
                        break;
                    }

                    outputStream.Write(buffer, 0, bytesRead);
                }
            }
        }

FileStreamResult will dispose your stream after writing it to response.OutputStream;

If you don't have to proxy your downloads through the website (security) you can generate Shared Access Signature and redirect your user straight to the storage. See here

This solves the exception, so thanks. But I now have the problem that once the `FileStreamResult` is returned the browser is not launching the save file as dialog. How can I get this to occur? I got this method from http://stackoverflow.com/a/19026034/626442. — MoonKnight, Nov 26 '14 at 14:20

DownloadRangeToStreamAsync and File Save using ASP.NET and MVC5

1 Answers1